[IA64-SGI] Recursive flags do not work for selective builds

[IA64-SGI] Recursive flags do not work for selective builds

arch/ia64/sn/Makefile sets CPPFLAGS, expecting that setting to
propogate to all the subdirectories.  For a normal build with its
recursive descent it does work, but doing a selective build like
'make arch/ia64/sn/kernel/io_init.i' does not do a recursive descent,
it goes directly to arch/ia64/sn/kernel/Makefile so the flags do not
get set.

To support selective builds, set the flags in all the subordinate Makefiles.

Signed-off-by: Keith Owens <kaos@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>